DTIC ADA551139: Data Assimilation and Model Simulations in the California Current

5 Pages·0.03 MB·English

by  Defense Technical Information Center

Data Assimilation and Model Simulations in the California Current
Robert L. Haney
Department of Meteorology (MR/Hy)
Naval Postgraduate School
589 Dyer Rd., Room 254
Monterey, CA 93943-5114
Phone: [phone]
Fax: [phone]
e-mail: [email protected]
Award #: N0001498WR30093
LONG-TERM GOALS
The long-term goal of my research is to develop, verify, and apply numerical ocean prediction models to
eastern boundary coastal regions in order to improve our scientific understanding of the structure and
dynamics of such regions.
OBJECTIVES
The broad objective of this research is to aid in the development of a reliable modeling capability for
eastern boundary current regions. My specific objective is to carry out and extensively verify several
DieCAST model simulations of the annual cycle in the California Current. I also wish to evaluate and
apply digital filter initialization (DFI) as a diagnostic tool in numerical ocean prediction.
APPROACH
I intend to carry out increasingly complex numerical simulations of the California Current using the
DieCAST regional model. I will then verify the simulations against ONR’s extensive eastern boundary
current (EBC) data sets. I also intend to evaluate iterative versions of digital filter initialization (DFI) as
a diagnostic tool for ocean analysis, and then apply the method to several quasi-synoptic hydrographic
data sets from the California Current and the Alboran Sea. Results from the California Current will be
compared with those obtained using the iterated geostrophic method (IGM, Allen 1993) and the results
from the Alboran Sea will be verified using direct measurements of the vertical velocity determined by
quasi-isobaric RAFOS floats.
WORK COMPLETED
I have adapted the DieCAST A-grid, low-dispersion (4th order) regional model (Dietrich 1997) to the
California coastal domain and completed two six-year long numerical simulations. Lateral boundary
conditions are prescribed from monthly climatological data and from a global ocean model simulation.
One of the simulations uses a climatological annual cycle of wind stress forcing, while the other
simulation includes an additional, idealized wind stress enhancement near each coastal headland
representative of observations (e.g., Enriquez and Friehe 1995; Rogers et al. 1998). Preliminary results
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1998 2. REPORT TYPE 00-00-1998 to 00-00-1998
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Data Assimilation and Model Simulations in the California Current
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Naval Postgraduate School,Department of REPORT NUMBER
Meteorology,Monterey,CA,93943
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have been published in an NPS Masters Thesis (Donato 1998), while a complete analysis of the
simulations is still in progress.
I have applied iterative forms of DFI to several idealized test cases and have determined an optimal
version of the method for use in diagnostic studies. DFI is now being applied to data sets from the
California Current (ONR’s CTZ and EBC programs) and the Alboran Sea (EC’s OMEGA project).
I contributed to two studies of the Alboran Sea that were published this year, and I collaborated in a
study of stochastic resonance in the thermohaline circulation that we recently submitted for publication.
RESULTS
Using climatological forcing, our DieCAST model simulation reproduces many of the main features of
the observed annual cycle of the California Current including the strengthening of the coastal jet in
spring and the weakening of the jet in autumn and winter. Coastal eddies in the simulation form
primarily off the major headlands, especially Cape Mendocino and Point Arena. As a result, a region of
maximum eddy kinetic energy (EKE), originally formed in the upper ocean over the continental slope in
late spring, migrates westward on a seasonal time scale. At the same time, the EKE spreads vertically
into the deep ocean, decreasing the EKE west of about 126W. This result identifies a non-dissipative
process that can account for the pronounced decrease of surface EKE west of 126W recently
documented in the literature (Kelly et al. 1998, Strub and James 1998). The additional forcing by the
headland wind jets is found to produce both local and remote changes to the simulated annual cycle.
The optimal form of DFI was shown to be able to diagnose the 3-dimensional circulation and vertical
velocity in an idealized growing baroclinic frontal wave with a relative accuracy greater than 90%. At
the same time, it is shown that accurate measurements of the currents are needed in order to diagnose
the mesoscale circulation on space scales as short as the Rossby radius. Using DFI, a closed cyclonic
eddy observed off Point Arena California was diagnosed to exhibit relatively small vertical velocities on
the scale of the eddy itself, but to have very significant vertical velocities (w~30m/d at 100 m)
associated with sub-eddy scale meanders in the otherwise circular flow around the eddy.
In the Alboran Sea studies, we used remotely sensed and in situ data to document a new mode of
synoptic scale variability in the Western Alboran Gyre and to interpret the variability dynamically. In our
thermohaline circulation study we showed that a small periodic variation in the fresh water fluxes, as
small as 5% of the mean fresh water flux, in the presence of background variability (noise), can induce
quasi-periodic transitions between two stable states of the thermohaline circulation.
IMPACT/APPLICATIONS
Our California Current annual cycle simulations are among the most advanced ones to date. Because of
the excellent observational data sets being used, the model verifications will represent the standards
against which other models will be evaluated in the future.
TRANSITIONS
Our research on data assimilation and simulations in the California Current is in broad support of the
efforts at FNMOC (M. Clancy) and NRL-Stennis (J. Kindle) to develop a real-time ocean analysis and
forecasting capability, including biology, for this region. This support manifests itself in close
coordination, and timely information exchange on such topics as model properties, experimental set-up,
forcing data, regional modeling difficulties, comparison and exchange of results, methods of model
verification, and so forth.
The DFI methodology has been accepted as the major diagnostic tool for estimating the ageostrophic
circulation and vertical motion in the EC-sponsored OMEGA project. This multinational European
project completed six quasi-synoptic oceanographic surveys in the western and central Mediterranean
last year.
RELATED PROJECTS
I am collaborating with J. Barth (OSU) in his application of the DFI diagnostic method to the EBC
synoptic survey data.
I am collaborating with D. Dietrich (MSU-CAST) in a DieCAST model hindcast of the Santa Barbara
Channel driven by high resolution wind stress data from COAMPS. I am also providing the COAMPS
wind stress data to Leo Oey for a similar SBC hindcast using the Princeton Ocean Model (POM).
I am collaborating with A. Viudez and J. Tintore (UIB, Spain) in diagnostic studies of mesoscale
variability in the Alboran Sea and modeling studies of climatic variability of the thermohaline
circulation.
REFERENCES
Allen, J. S., 1993: Iterated geostrophic intermediate models. J. Phys. Oceanogr., 23, 2447-2461.
Dietrich, D. E., 1997: Application of a modified Arakawa “A” grid ocean model having reduced
numerical dispersion to the Gulf of Mexico circulation. Dyn. Atmos. and Oceans, 27, 201-217.
Donato, J., 1998: The seasonal cycle of mesoscale variability in the California Current: A DieCAST
model simulation. M. S. Thesis, Department of Meteorology, Naval Postgraduate School, Monterey,
CA, Sep. 1998, 68pp.
Enriquez, A. G., and C. A. Friehe, 1995: Effects of wind stress and wind stress curl variability on
coastal upwelling. J. Phys. Oceanogr., 25, 1651-1671.
Kelly, K. A., R. C. Beardsley, R. Limeburner, K. H. Brink, J. D. Paduan and T. K. Chereskin, 1998:
Variability of the near-surface eddy kinetic energy in the California Current based on altimetric, drifter
and moored current data. J. Geophys. Res., 103, 13067-13083.
Rogers, D. P., C. E. Dorman, K. A. Edwards, I. M. Brooks, W. K. Melville, S. D. Burk, W. T.
Thompson, T. Holt, L. M. Strom, M. Tjernstrom, B. Grisogono, J. M. Bane, W. A. Nuss, B. M. Morley
and A. J. Schanot, 1998: Highlights of Coastal Waves 1996. Bull. Am. Meteor. Soc., 79, 1307-1326.
Strub, P. T. and C. James, 1998: Altimeter-derived variability of surface velocities in the California
Current system: 2. Seasonal circulation and eddy statistics. Deep Sea Res. (in press).
PUBLICATIONS
Donato, J., 1998: The seasonal cycle of mesoscale variability in the California Current: A DieCAST
model simulation. M. S. Thesis, Department of Meteorology, Naval Postgraduate School, Monterey,
CA, Sep. 1998, 68pp.
Viudez, A., R. L. Haney and J. Vazquez-Cuervo, 1998: The deflection and division of an oceanic
baroclinic jet by a coastal boundary: A case study in the Alboran Sea. J. Phys. Oceanogr., 28, 289-303.
Viudez, A., J.-M. Pinot and R. L. Haney, 1998: On the upper layer circulation in the Alboran Sea. J.
Geophys. Res., 103, 21653-21666.
Viudez, A., R. L. Haney and J. T. Allen, 1998: A study of the balance of horizontal momentum in a
vertical shearing current. J. Phys. Oceanogr. (submitted).
Shearman, R. K., J. A. Barth, J. S. Allen and R. L. Haney, 1998: Diagnosis of the three-dimensional
circulation in mesoscale features with large Rossby number. J. Phys. Oceanogr. (submitted).
Velez, P., A. Alvarez, P. Colet, J. Tintore and R. L. Haney, 1998: Stochastic resonance in the global
ocean conveyor belt. Climate Dynamics (submitted).
Haney, R. L., 1997: Global warming? Invited presentation, Annual Climate Diagnostics and Prediction
Workshop, E. O. Lawrence Berkeley National Laboratory, Berkeley. CA, 6-10 Oct., 1997.
Haney, R. L., 1998: The general circulation in the California Current: Observations and simulations.
Invited presentation, Akio Arakawa Fest: Symposium on General Circulation Model Development:
Past, Present, and Future, UCLA, 20-22 Jan., 1998.
Haney, R. L., D. E. Dietrich and R. A. Hale, 1998: The role of headland wind jets in the annual cycle of
the California Current system. Paper presented at the 1998 Ocean Sciences Meeting, San Diego, CA, 9-
13 Feb., 1998.